A conventional thermoelectric device is a semiconductor device that converts a temperature difference into electricity or vice versa. The most common thermoelectric device is the thermoelectric generator (TEG), which converts a temperature difference into electricity, and which is composed of semiconductor legs, p-type and n-type legs connected in series electrically, and in parallel thermally. The thermoelectric material in TEGs is generally characterized with three fundamental properties: a high electrical conductivity (σ), a large Seebeck coefficient, α and a low thermal conductivity (λ). Those properties are gathered in the so-called thermoelectric figure-of-merit (ZT), where ZT=σα2T/λ, and Z is a measure of a material's thermoelectric properties, T is the absolute temperature. The Seebeck coefficient is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference across that material, which may also be called the thermopower or thermoelectric power of a material. In order to achieve a thermoelectric material with high efficiency, the material should preferably have both high thermodiffusion and low thermal conductivity at the same time. However, it is difficult to find materials with these characteristics. The importance of the material properties, its ZT values, is due to the fact that it is directly connected with the efficiency of the thermoelectric devices. High ZT materials give efficient devices.
The thermoelectric cooler, which pumps heat from one side to the other side to create a temperature difference thanks to an electrical power, is also a thermoelectric devices, called Peltier cooler.
Currently, thermoelectric devices are mostly used as for example in Peltier coolers, temperature sensors However, they are regarded to have great potential for electricity production from waste heat and natural heat source (geothermal, solar) in the future, but in order to be used as large-area heat exchangers or in combination with large area solar cells, there is a need to develop TEGs which are suitable for large areas and suitable for low-temperature applications (below 200° C.).
Several families of materials have been considered as thermoelectric materials: semi-metals, metal-oxide, and inorganic semiconductor materials. Up to now, the best thermoelectric materials for use at temperatures up to 200° C. are heavy metal alloys composed of low natural abundance, such as for example Bi2Te3. This material is used since it has both high Seebeck effect and low thermal conductivity. The thermoelectric figure-of-merit is close to 1. However, it would be very expensive to create a large area heat exchanger from this material. In addition, the toxicity is a disadvantage.
There are only few studies on the use of organic materials for thermoelectric applications since organic materials have been regarded to have too low thermoelectric figure-of-merit to be interesting for use in thermoelectric devices. Recently, the thermoelectric properties of a modified conductive polymer has been investigated, see Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene) Bubnova et al, Nature Materials vol. 10, June 2011. Here, the p-type leg is made of PEDOT-Tos (PEDOT=Poly(3,4-ethylenedioxythiophene), Tos=tosylate) treated with tetrakis(dimethylamino)ethylene (TDAE) and the n-type leg is obtained from an organic salt TTF-TCNQ. Both legs are electrically connected to a top Au electrode. The thermoelectric figure-of-merit for this conducting polymer was found to be 0.25.
However, there is still a great demand for TEGs for low temperature applications (below 200° C.) having high thermoelectric figure of merit and which can be produced at low cost and still be environmentally friendly.